Wide-field-of-view (WFOV) imaging optical systems are used in a number of applications. For example, surveillance systems, helmet-mounted sensors, missile warning sensors for aircraft, and space and aircraft acquisition sensors all may use WFOV optical systems. In this context, a WFOV optical system typically seeks a field of view of at least 60 degrees in at least one dimension.
Most WFOV imaging optical systems are of the inverse telephoto or fisheye type. The inverse telephoto optical system has a front (that is, nearest the scene) lens or lens group with a negative optical power, in order to view a wide field of view, and a rear (that is, nearest the eye or the sensor) lens or lens group with a positive optical power to focus the light rays onto the focal plane. This distribution of optical powers is necessary to minimize the angle of incidence of the chief ray, thereby minimizing the aberration. The associated effect, however, is that the incoming beam first diverges when it passes through the negative-optical-power front lens or lens group. The additional lenses required to focus the light beam to the focal plane result in an overall length of the optical system that is typically 3–8 times the effective focal length. It is therefore difficult to design a compact, light-weight WFOV optical system. Additionally, the numerical aperture of the inverse telephoto lens is typically relatively small, resulting in a low signal-to-noise ratio and a required long exposure time to produce a usable image.
There is a need in many applications for a WFOV imaging optical system that is more compact and lighter in weight than the inverse telephoto lens. The present invention fulfills this need, and further provides related advantages.